1. Field of the Invention
This invention relates to techniques for heating glass sheets for the purpose of a thermal treatment by rapid cooling of the thermal toughening type--or by slow cooling of the annealing type--optionally associated with shaping. These heating techniques are preferably applied to installations intended for the production of curved glazing for automobile vehicles.
2. Description of the Related Art
The hot treatments for glass sheets supported in a horizontal position, such as those listed above, demand very strict control of the heating of the glass. A temperature deviation of about ten degrees within the glass, a value that can be compared with the exit temperature from the furnace which itself exceeds 600.degree. C., is sufficient to cause a modification in the performance of the glass and to generate defects, notably variations in the curvature after the shaping step. To this accuracy required for heating, there must be added also the need, in this case of an economic nature, for production at high output rates, which presupposes that the dwell time of the glass sheets in the furnace shall be limited to the strict minimum. As a general rule, the furnaces are of the type known as straight-through, that is to say the glass sheet passes through them from one end to the other, while being kept moving by means provided for causing the glass sheet to pass through the various heating units. In vertical installations, these means are essentially composed of tongs that travel along a rail. These tongs enable the position of the glass in the furnace to be controlled very accurately, which represents an important advantage where the following step consists of a curving operation, especially if the desired shape is complex, the curving then most commonly comprising at least one pressing phase. These tongs, however, leave marks that are unacceptable according to the highest quality standards, which are almost always required by present-day automobile manufacturers.
In these conditions, the installations of the horizontal type are gradually being replacing those of the vertical type. In horizontal-type installations, the means for moving the glass sheets through the furnace are, generally, composed of motor-driven rollers on which the glass sheets rest in a horizontal or substantially horizontal position, or equivalent conveying means.
In European Patent Application EP-A-241 356 there is described, for example, a furnace having two, i.e., upper and lower, heating layers composed of electrical resistors placed on either side of the path followed by the glass sheets, each layer being divided into several independent longitudinal zones, themselves divided into transverse sub-zones which are regulated independently and disposed in such a manner that the separating spaces between the transverse sub-zones do not constitute "cold" bands parallel to the direction of the glass sheets. The electrical resistors are formed of wires wound around silica tubes, these tubes being oriented parallel to the width of the furnace, or in other words perpendicular to the general direction of movement of the glass sheets passing through the furnace. Such an arrangement of the resistors is especially well adapted to the uniform heating of the glass sheets or to a differentiated heating of the central parts and of the edges of the glass sheet. However, this differentiation of temperatures is possible only for fairly wide portions of the area, each portion of the glass sheet covering, for example, at least 10% of the total area. Since the quality requirements are gradually evolving towards the total elimination of marks from tongs, this type of horizontal furnace is being employed also for those cases where shaping takes place to very small radii of curvature with the virtual formation of folds, notably those that are most difficult to produce, that is folds localized near the edges of the glass sheets.
In these cases, it is no longer a matter of differentiated heating of a whole portion of the area. In contrast, it becomes necessary to control virtually point by point the temperature of the glass sheets; this for the purpose of defining, for example, fold lines needing extra heating in order to locally increase the plasticity of the glass.
A known earlier method of localized extra heating consists of covering the surface of the glass sheet, along the fold line, with a black object such as carbon powder, so that the glass locally absorbs more heat. The deposition of this black object, however, requires additional equipment, and furthermore the extra heating is dictated by the temperature of the remainder of the glass sheet and by the nature of the black object, and so there is a lack of flexibility.
It is also known to use extra heating means integral with the furnace and not on the glass sheets. In U.S. Pat. No. 4,441,907 it has been proposed to use gas nozzle pipes mounted on a movable carriage which moves parallel to the direction of movement of the glass sheets: when the gas nozzle pipes come into action, generally in the downstream part of the path of the glass sheet, they are fixed relative to the sheet, thus enabling a narrow band of the glass surface to be preferentially heated. On the other hand, this system requires a precision mechanical system for perfectly synchronizing the movements of the glass and the carriage, and for operating reproducible controls for positioning the gas nozzle pipes. Furthermore, the time during which a carriage with its gas nozzle pipes is immobilized is very long (the time for additionally heating the glass and the time for returning to the starting position). The system known from U.S. Pat. No. 4,441,907 therefore requires a very wide spacing between two glass sheets.
There has also been proposed, in German Patent Application DE-A-37 42 481 a device enabling bands parallel to the direction of feed of the glass to be given additional heating locally. For this purpose, the furnace is equipped with additional heating means composed of heating bars extending essentially parallel to the length of the furnace and suspended from the ends of rods which penetrate to a greater or lesser extent into the furnace and perpendicularly to its length. Progressively as the glass sheet passes through the additional heating zone, the bars are moved further apart--or in contrast brought nearer together--so that a continuous band of the glass sheet is additionally heated. With an extra heating device of this type, the dead time between two panes is advantageously less than with the device described above. Nevertheless, this device does require a furnace sufficiently wide to permit the introduction of the glass sheets across the furnace, in order that the shorter sides--along which the fold lines are disposed in the majority of models--are well situated parallel to the longitudinal direction of the furnace.
Moreover, these localized extra heating devices, known from U.S. Pat. No. 4,441,901 and German DE-A-37 42 481, comprise many elements which are cantilever mounted. In view of the temperature of the furnace, these elements are therefore very sensitive to deformation, with the consequence that the heating parameters for the glass sheets may change in the course of time and adjustments may need to be made for each series of panes, even though this series has already been produced on the line.